How do you create eTags?

[Egg Bramhill]

I often use Pingdom to check on a websites page to check it's download score but don't get hung up to much about the results as often many are out of your hands if using Xara to create a site. One thing that does niggle me is the 'Configure eTags' warning.

I've done numerous searches on how I can create eTags but never found one single method of doing so. No big sweat, just out of interest.

Anyone any idea re this?